Female rats poisoned with multiple LD50s of soman or tabun have been shown previously to respond to the protective effects of HI-6 more positively than male rats. This present study was designed first to determine the distribution pattern and concentration of [14C] HI-6 in rats, and secondly, to determine the possibility that HI-6 might be located in high concentrations in critical tissues in the female as opposed to the male. To these ends, [14C] HI-6 was administered to groups of male and female rats and its radiolabelled distribution determined by whole body autoradiography and/or by measurement of its actual concentration, by scintillation spectrometry. The experiments were repeated in the presence of 2 x LD50 soman and supporting therapy with atropine. In both sexes, HI-6 levels were highest in the kidney, followed in order by cartilage greater than plasma greater than liver greater than heart greater than or equal to lung greater than or equal to diaphragm greater than brain and spinal cord. The relative distribution in the two sexes was confirmed by both methods and was not significantly altered in the presence of soman and atropine. The lack of a measurable difference in tissue distribution of [14C] HI-6 derived radioactivity between males and females suggested that the hormone-dependent difference in the protective effects previously observed was not due to selective accumulation of [14C] HI-6 in organs believed to be important in its therapeutic activity, such as brain or diaphragm.